The present invention relates to a variable-gain amplifier circuit, an offset control method in a variable-gain amplifier circuit, a radio receiver with a variable-gain amplifier circuit, and a radio receiving method in a radio receiver with a variable-gain amplifier. More specifically, the invention relates to a variable-gain amplifier circuit having a DC offset correcting function, a method for correcting a DC offset component in this circuit, a radio receiver with a variable-gain amplifier circuit having a DC offset correcting function, and a radio receiving method for receiving a radio frequency signal while correcting a DC offset component in this receiver.
As a conventional variable-gain amplifier circuit, for example, a circuit as shown in FIG. 1 is known. In this figure, a variable-gain amplifier circuit comprises: an operation amplifier circuit 1 for receiving an input signal, for example, a radio or intermediate frequency signal, as a phase input to output an amplified signal; a variable attenuation circuit 2 for setting a desired quantity of variable attenuation corresponding to a gain of the output of the operation amplifier circuit 1 to supply the set quantity of variable attenuation to an anti-phase input; an input terminal 6 for receiving the input signal; and an output terminal 7 for outgoing the amplified output.
In this variable-gain amplifier circuit, assuming that the gain of the operation amplifier circuit 1 is g and the transmittance of the variable attenuation circuit is f, the transmittance A of the whole circuit can be expressed by the following formula (1). EQU A=g/(1+gf) . . . (1)
Assuming that the value gf is sufficiently larger than 1, the formula (1) can be approximated to A=1/f. In the circuit of FIG. 1, it is possible to control the value f and to allow the gain A which is the reciprocal number thereof, to be variable. As an embodied construction of the variable attenuation circuit, the circuits of FIGS. 12 and 13 which will be described later are known. That is, in accordance with a potential difference between the terminal of the variable attenuation circuit 2 on the side of the output terminal 7 and the terminal of the operation amplifier circuit 1 on the negative phase input side, a certain rate of electric potential occurs between the terminals. By selecting this rate by means of switches, the circuit functions as a variable attenuation circuit.
In such a variable attenuation circuit, there is a problem in that the offset of the operation amplifier circuit 1 appears in the output. In addition, the offset fluctuates in accordance with the switching of the gain. For example, when the input offset voltage of the operation amplifier circuit 1 is 2mV and the voltage gain of the variable-gain amplifier circuit is 10, a DC offset voltage of 20mV appears in the output. When the voltage gain is 100, the DC offset voltage is 200mV. In recent communication appliances, there is a tendency to set a lower supply voltage in order to decrease consumed power. For example, assuming that the supply voltage is 3 V, an offset voltage of 200 mV can not be disregarded.
In addition, the following conventional radio receiver is known. The receiver controls signal levels in first and second intermediate-frequency amplifier circuits. In this case, since the signal frequency is apart from direct current even if the DC offset fluctuates, a desired signal can be easily separated from the offset by means of a capacity coupling circuit or the like. However, there are many parts which are difficult to be integrated as a semiconductor integrated circuit such as an intermediate-frequency band-pass filter (BPF), so that the aforementioned receiver is not suitable for a small receiver.
In addition, a receiver of a direct conversion system for directly converting a high frequency signal to a baseband signal without passing through an intermediate frequency signal has been proposed. This system is suitable for a small receiver since the number of parts which are difficult to be integrated is less than the of the aforementioned receiver. However, since there is no intermediate frequency, the baseband section must have a great gain, so that the offset is amplified. In addition, since the gain must be controlled by the baseband in order to obtain a signal of a suitable level, the magnitude of offset fluctuates. Since the baseband signal contains a signal component of direct or nearly direct current, the DC offset and its fluctuation are the same as those of a jamming wave overlapping with a desired signal in a digital signal processing section, so that the quality of communication may be deteriorated.